The present invention relates generally to semiconductor integrated circuits, and more particularly, to a system and method to improve image sensor sensitivity of optical sensors in integrated circuit designs.
Semiconductor integrated circuits (ICs) are now produced that can have sensor functions as camera chips. For embodiment, the camera chip is mounted in the focal plane of a camera, and light image information is sensed and converted into electrical data for reconstructing the original image. Such a sensor IC is constructed in silicon, and has both logical and optical sensor functions. It is understood that silicon has a predictable light spectral response, which is different from that of the human eye. Silicon photoresponse is greatest in the red end of the visible light spectrum (with longer wavelengths), while the human photoresponse is greatest in the middle green part of the visible light spectrum. Silicon photoresponse shifts even further toward the red (i.e. towards longer wavelengths) with increasing depth of the photosensitive junction. In order to enhance the blue and green photoresponse of a silicon device, the diffused junction is typically very shallow, and very close to the surface.
The production of a complex, but practical, IC logic chip requires intricate multilevel metal interconnection structures. The integrated circuit must contain logic circuitry for control and for signal routing and decoding. The production of complex multilevel metal interconnections between the active surface of the logic portion of the IC and bond pads that connect to external circuits require via etches that are difficult to control precisely. For instance, a part of the complexity is a deep, high aspect ratio via etching through dielectric oxide layer. The precision of control of the etch depth of many vias placed across a large round silicon wafer is an important requirement. A production-worthy technique that achieves that precision is the imposition of an etch stop layer. To subsequently etch through the etch stop layer requires a different etchant. The material that is used for the etch stop layer is typically silicon-oxynitride (SiON).
However, SiON does not transmit all colors of the visible spectrum equally to the optical sensor portion of the IC. The spectral response of any silicon photodetector is therefore decayed by the presence of such a SiON etch stop layer. Conflicting requirements are presented for the production of a solid-state image sensor that can present a colored image that is suitable for the human eye.
Desirable in the art of solid-state image sensor designs are improved designs that improve spectral transmission even with an etch stop layer being deployed.